The present invention relates to a cathode ray tube having an electron gun employing an indirectly heated cathode, and in particular to a cathode ray tube having reduced a power consumption of a heater serving as a heating element of the indirectly heated cathode.
Cathode ray tubes such as TV picture tubes and display tubes are widely used as a display means in various kinds of information processing equipment because of their capability of high-resolution image reproduction.
The cathode ray tubes of this kind include an evacuated envelope comprising a panel portion having a phosphor screen formed of phosphors coated on its inner surface, a neck portion and a funnel portion for connecting the panel portion and the neck portion, an electron gun housed in the neck portion comprising an electron beam generating section including an indirectly heated cathode, a control electrode and an accelerating electrode, and a main lens section formed of plural electrodes for focusing and accelerating an electron beam generated in the electron beam generating section toward the phosphor screen, and a deflection yoke mounted around the funnel portion for scanning the phosphor screen with the electron beam emitted from the electron gun.
FIG. 6 is a schematic cross-sectional view of a shadow mask type color cathode ray tube for explaining an example of a structure of a cathode ray tube. Reference numeral 1 denotes a panel portion, 2 is a funnel portion, 3 is a neck portion, 4 is a phosphor screen formed of phosphors coated on the inner surface of the panel portion 1, 5 is a shadow mask serving as a color selection electrode, 6 is a magnetic shield for shielding an external magnetic field (the Earth""s magnetic field) for preventing the Earth""s magnetic field from changing the trajectory of electron beams. Reference numeral 7 denotes a deflection yoke, 8 is external magnets for beam adjustment, 9 is an electron gun provided with indirectly-heated cathodes for emitting three electron beams and 10 are the three electron beams only one of which is shown.
The three electron beams 10 from the electron gun 9 are modulated by video signals from an external signal processing circuit (not shown), respectively, and are projected toward the phosphor screen 4. The electron beams 10 scan the phosphor screen 4 two-dimensionally by being subjected to the horizontal and vertical deflection magnetic fields generated by the deflection yoke 7 mounted around the transition region between the neck portion 3 and the funnel portion 2. The shadow mask 5 reproduces a desired image by passing the three electron beams through a large number of apertures therein to the phosphor screen such that each beam impinges upon and excites only one of the three kinds of color phosphor elements in the phosphor screen.
FIG. 7 is a side elevation view of the electron gun 9 for explaining an example of a structure of the electron gun 9 used for the color cathode ray tube shown in FIG. 6. The electron gun 9 comprises a control electrode (the first grid electrode or G1) 11, an accelerating electrode (the second grid electrode or G2) 12, focus electrodes (the third grid electrode or G3, the fourth grid electrode or G4, and the fifth grid electrode or G5) 13, 14, 15, an anode (the sixth grid electrode or G6) 16, and a shield cup 17 physically retained in axial predetermined spaced relationship in the order named by multiform glasses 20, and the respective electrodes are electrically connected to respective stem pins 18a implanted in a stem 18 by welding to the stem pins 18a a tab or a lead provided to the electrodes.
In this electron gun 9, an indirectly heated cathode structure 21 is spaced closely from the electron beam apertures in the control electrode 11 toward the stem 18, and has heaters for heating the electron-emissive layers.
Reference numeral 19 denote bulb spacer contacts for centering the central longitudinal axis of the electron gun 9 coincident with the axis of the neck portion 3 by pressing resiliently against the inner wall of the neck portion 3 and for effecting delivery of an anode voltage from the internal conductive coating coated on the inner walls of the funnel portion 2 and the neck portion 3 to the electron gun 9.
The indirectly heated cathode structure 21, the control electrode 11 and the accelerating electrode 12 form an electron beam generating section (a triode portion). The focus electrodes 13 to 15 accelerate and focus the electron beams emitted from the electron beam generating section, and then a main lens formed between the focus electrode 15 and the anode 16 focuses the electron beams onto the phosphor screen.
The stem 18 is fused to close the open end of the neck portion 3 of the vacuum envelope, and signals and voltages from external circuits are applied to the respective electrodes via the stem pins 18a. The external magnets 8 (a magnet assembly) for beam adjustment shown in FIG. 6 correct errors in landing of the electron beams on the phosphor picture elements caused by a delicate misalignment in axis or a delicate rotational error between the electron gun 9 and the panel portion 1, the funnel portion 2 and the shadow mask 5.
FIG. 8 is a cross-sectional view of the indirectly heated cathode structure 21 shown in FIG. 7. The indirectly heated cathode structure 21 comprises bead supports 22, an eyelet 23, heater supports 24, a heater 25, a base metal 27 for supporting an electron-emissive material 26, a cathode support sleeve 28 and a cathode cylinder 29.
The indirectly heated cathode structure 21 is fixed on multiform glasses 20 by the eyelet 23 and the bead supports 22. The heater 25 housed within the cathode support sleeve 28 are fixed by welding its ends (leg portions) to the heater support 24.
FIGS. 9A and 9B are illustrations of a structure of the heater 25, FIG. 9A being a side view of the heater 25 and FIG. 8B being an enlarged fragmentary cross-sectional view of the encircled portion designated xe2x80x9cAxe2x80x9d in FIG. 9A. As shown in FIG. 9B, the heater 25 comprises a tungsten wire 31 spirally wound, an alumina insulating layer 32 coated around the tungsten wire 31, and a blackened fine-powder tungsten layer 33 coated around the alumina insulating layer 32. The blackened layer 33 is intended for lowering the temperature required of the heater 25 by improving the heat radiation from the heater 25, and consequently improving the reliability of the heater 25.
In FIG. 9A, reference character HT denote leg portions of the heater 25 comprised of tungsten wires spirally wound in three layers, HD is a major heating portion of the heater 25 formed by winding spirally in a large diameter a tungsten coiled wire having been wound initially spirally in a small diameter (hereinafter referred to merely as a coiled coil portion), HA is a portion coated with alumina, HB is a blackened portion covered with the blackened fine-powder tungsten layer 33, HE are portions not covered with alumina and reference numeral 39 in FIG. 9B denotes a hollow formed after dissolving and removing a molybdenum mandrel.
A method of forming the leg portions HT of the heater by winding tungsten wires in three layers is disclosed in Japanese Patent Application Laid-open No. Hei 11-354041 (laid-open on Dec. 24, 1999).
FIGS. 10A-10E illustrate sequence of steps in a conventional method of fabricating the conventional heater.
In FIG. 10A, a tungsten wire 31 is wound spirally forward as indicated by an arrow P around a molybdenum mandrel wire 40 up to point A.
Next, as illustrated in FIG. 10B, the tungsten wire 31 is wound spirally backward from point A to point B as indicated by an arrow Q.
Then, as illustrated in FIG. 10C, the tungsten wire 31 is wound spirally forward again from point B to point C over a centerline CL for folding in a subsequent process, as indicated by an arrow R, forming a three-layer winding portion TWA ranging from point A to point B.
Next, as illustrated in FIG. 10D, the tungsten wire 31 is wound spirally backward from point C to point D as indicated by an arrow S.
Next, as illustrated in FIG. 10E, the tungsten wire 31 is wound spirally forward again from point D to point E as indicated by an arrow T, forming a three-layer winding portion TWB ranging from point C to point D.
The tungsten wire thus wound around the molybdenum mandrel wire 40 is cut at the respective centers F, G of the three-layer winding portions TWA and TWB to provide a tungsten wire winding having a length HQL for one heater with the leg portions TWLA, TWLB of three-layer winding, and the tungsten wire winding of the length HQL is formed into a final shape by folding the length HQL in two halves at the centerline CL and twisting the two halves around each other as shown in FIG. 9A. Then, the molybdenum mandrel wire 40 is dissolved with acid, leaving the hollow 39 as shown in FIG. 9B.
The heater having the leg portions of the above three-layer winding structure provides the following advantages:
(i) prevention of breaks of a tungsten wire by sparks within a cathode ray tube,
(ii) reduction of power consumption by concentration of heat generation in the coiled coil portion HD (see FIG. 9A) due to low resistance of the three-layer winding portions and resultant reduced heat generation,
(iii) improvement in workability in the operation of welding the heater,
(iv) suppression of heat generation in the portions not covered with alumina caused by an overcurrent upon power turn on.
Incidentally, in referring to the number of winding layers, an n-layer winding, or an n-layer structure can also be used in addition to xe2x80x9cwound inn layers, in this specification.
The tungsten wire used for heaters are very thin, and are usually 30 xcexcm to 50 xcexcm in diameter. The structure of the wound thin wires is very weak in mechanical strength, and welding of heaters to a heater support requires a great deal of skill. The three-layer winding structure improves workability in welding heaters, and suppresses occurrences of breaks of heaters by sparks or overcurrents upon power turn on.
In the above-explained heater, consideration has been given to reduction of power consumption and workability in welding, but recently further reduction of power consumption is needed in view of energy saving.
There is a limit to reduction of the heater power consumption obtained by forming the heater leg portions by winding in plural layers only, because reduction of electrical resistance by layer shorts is not great.
It is an object of the present invention to provide a cathode ray tube provided with an indirectly heated cathode structure having reduced its power consumption by reducing electrical resistances of its heater leg portions without deteriorating workability in welding.
To achieve the above object, in accordance with an embodiment of the present invention, there is provided a cathode ray tube comprising: an evacuated envelope comprising a panel portion, a neck portion, a funnel portion for connecting the panel portion and the neck portion and a stem having a plurality of pins therethrough and being sealed to close the neck portion at one end thereof; a phosphor screen formed on an inner surface of the panel portion; an electron gun housed in the neck portion, the electron gun comprising an electron beam generating section including an indirectly heated cathode structure having a heater therein, a control electrode and an accelerating electrode, and a plurality of electrodes disposed downstream of the electron beam generating section for focusing and accelerating an electron beam emitted from the electron beam generating section toward the phosphor screen; and a deflection yoke mounted externally around the funnel portion for scanning the electron beam on the phosphor screen; the heater comprising a major heating portion having a spirally wound heating wire and two leg portions connected to opposite ends of the major heating portion, the two leg portions being welded to electrical conductors for applying voltages thereto at portions in the vicinity of open ends of the two leg portions, respectively, the heater being covered with an insulating film except for the portions for welding, the two leg portions comprising at least five layers of winding formed by spirally winding heating wires identical with the heating wire of the major heating portion, and numbers of turns per unit length in each of the at least five layers of winding in the two leg portions being smaller than a number of turns per unit length of the heating wire of the major heating portion.
To achieve the above object, in accordance with another embodiment of the present invention, there is provided a cathode ray tube comprising: an evacuated envelope comprising a panel portion, a neck portion, a funnel portion for connecting the panel portion and the neck portion and a stem having a plurality of pins therethrough and being sealed to close the neck portion at one end thereof; a phosphor screen formed on an inner surface of the panel portion; an electron gun housed in the neck portion, the electron gun comprising an electron beam generating section including an indirectly heated cathode structure having a heater therein, a control electrode and an accelerating electrode, and a plurality of electrodes disposed downstream of the electron beam generating section for focusing and accelerating an electron beam emitted from the electron beam generating section toward the phosphor screen; and a deflection yoke mounted externally around the funnel portion for scanning the electron beam on the phosphor screen; the heater comprising a major heating portion having a spirally wound heating wire and two leg portions connected to opposite ends of the major heating portion, the two leg portions being welded to electrical conductors for applying voltages thereto at portions in the vicinity of open ends of the two leg portions, respectively, the heater being covered with an insulating film except for the portions for welding, the two leg portions comprising at least three layers of winding formed by spirally winding heating wires identical with the heating wire of the major heating portion, numbers of turns per unit length in each of the at least three layers of winding in the two leg portions being smaller than a number of turns per unit length of the heating wire of the major heating portion, and the numbers of turns per unit length in each of the at least three layers of winding in the two leg portions being within a plus or minus variation of not greater than 30% in the at least three layers.
The present invention is not limited to the above structures, and various changes and modifications may be made without departing from the scope of the invention as defined in the appended claims.